At least several humoral factors appear to be involved in the regulation of mammalian hemopoiesis. For example, in erythropoiesis, both the hormone erythropoietin as well as a factor derived from cell conditioned media (termed 'burst-promoting activity,' BPA) are necessary for the full development of erythroid cell progenitors in vitro. However, in addition to erythropoietin, other factors may play an important role in regulating red blood cell production in vivo. Recently, we as well as others have demonstrated the effects of hemin, a normal hemoglobin derivative, in the augmentation of erythroid clonal cell growth in vitro. We have also observed the enhancement of erythroid cell growth in vivo following the administration of hemin to mice. It is the aim of this application to more fully assess the effects of hemin at early stages of hematopoietic cell development in order to determine the role of hemin in the commitment of pluripotent stem cells to the erythroid pathway of hematopoietic cell differentiation. Experiments will be conducted which will attempt to distinguish between hemin-induced enhancement of erythroid differentiation, cellular proliferation, or an effect on commitment itself. Both early committed erythroid cell progenitors (BFU-E) and the more primitive, pluripotent cell progenitors (CFU-GEMM) will be assayed for their responses to hemin. This will permit the distinction between an effect on committed erythroid cell progenitors (BFU-E) and the more-primitive uncommitted stages of hematopoietic cell development (CFU-GEMM). The effects of hemin will be assessed both in clonal cell cultures as well as in the intact animal. The latter model should help to establish hemin as a candidate 'physiological' regulator/modulator of erythropoiesis in the mammalian organism. In addition, the relationship of hemin to other factors involved in erythropoietic regulation in vitro (e.g., erythropoietin and BPA) will be assessed in detail. Since hemin is used therapeutically in several clinical disorders of erythropoiesis and neoplastic cell growth, its specific cellular effects in modulating hematopoietic cell growth in humans may be better understood as a result of these investigations.